Sol–gel derived bioactive glasses are thought to have high potential as materials for bone regeneration and drug delivery devices. They bond to bone and have a controllable degradation rate. Their unique nanoporosity provides high surface area and exposes a high concentration of surface hydroxyl groups. Protein adsorption, degradation rate and cellular response are known to be affected by nanotopography, therefore it is important to be able to produce glasses with a range of pore sizes. In this study, the modal nanopore diameters of glasses with the bioactive composition 70 mol% SiO2 and 30 mol% CaO (70S30C) were successfully increased from 12 to 30 nm by adding specific amounts of trimethylethoxysilane (TMES) during the sol–gel process. The mechanism of the nanoporosity modification was studied with transmission electron microscopy (TEM), nitrogen sorption and 29Si magic angle spinning (MAS) solid-state NMR spectroscopy. Solid-state NMR was used to investigate how the modification processes affected the atomic scale structure of the glass, such as Q structure and network connectivity, which was related to the changes in nanostructure using combinations of nitrogen sorption and TEM. The TMES was found to inhibit the fusion of the nanoparticle structural components of the glasses, causing an increase in pore size.